The color of the Martian sky and its influence on the illumination of the Martian surface

The dust in the atmosphere above the Mars Pathfinder landing site produced a bright, red sky that increases in redness toward the horizon at midday. T here is also evidence for an absorption band in the scattered light from th e sky at 860 nm. A model of the sky brightness has been developed [Markiewi cz et nl., this issue] and tested against Imager for Mars Pathfinder (IMP) observations of calibration targets on the lander. The resulting model has been used to quantify the total diffuse flux onto a surface parallel to the local level for several solar elevation angles and optical depths. The mod el shows that the diffuse illumination in shadowed areas is strongly redden ed while areas illuminated directly by the Sun (and the blue forward scatte ring peak) see a more solar-type spectrum, in agreement with Viking and IMP observations. Quantitative corrections for the reddening in shadowed areas are demonstrated. It is shown quantitatively that the unusual appearance o f the rock Yogi (the east face of which appeared relatively blue in images taken during the morning but relatively red during the afternoon) can be ex plained purely by the changing illumination geometry. We conclude that any spectrophotometric analysis of surfaces on Mars must take into account the diffuse flux. Specifically, the reflectances of surfaces viewed under diffe rent illumination geometries cannot be investigated for spectral diversity unless a correction has been applied which removes the influence of the red dened diffuse flux.